Rationale for femtosecond magnetism explored with x-ray core-hole excitation

In magnetic metals the core-hole excitation by x rays leads to a sudden creation of an impurity that is screened in a spin-dependent way. A scattering experiment in the L-3 region terminating with a 3s hole has already been shown to give information on the time scale of this screening process. Here we present a linear-rate model for the analysis of such experiments and apply this to Ni, Co, and Fe metals. This model yields quantitative results that could not be obtained before using an empirical model. The screening time constant in Ni (most likely similar to 1.5 fs) is definitely longer than in Fe, which presumably is caused by narrow-band effects. The screening time is 0.43 fs in Co, while 0.18 fs is an upper limit in Fe metal. In agreement with the experiment, the model clarifies the variation in the dichroism due to the competition between the core-hole decay and the time it takes to accumulate the screening charge. Finally, the perspectives of this approach are briefly discussed including the importance of future experiments on diluted magnetic impurities.